The present invention pertains to isolated genes encoding avian interleukin-15 and to purified interleukin-15 polypeptides.
Most chickens produced in developed countries for consumption and egg-laying (at least 10 billion per year) are vaccinated to protect them against Marek""s disease. All of the egg-laying chickens and breeder stocks are also vaccinated with Newcastle Disease Virus, Infectious Bursal Disease Virus, Infectious Bronchitis Virus, Fowlpox Virus and Coccidial vaccines. For optimal protection, Marek""s vaccination is performed either at or before hatching. One obstacle to the development of efficacious pre-hatching and at-hatching vaccination regimens is that the embryonic and newly hatched avian immune system is not fully developed and cannot mount as effective an immune response to the immunogen as at 2-3 weeks after hatching. Thus, there is a need in the art for agents and compositions that enhance the effectiveness of pre- and post-hatching avian vaccines.
Interleukin-2 and interleukin-15 are related cytokines that stimulate the activity and proliferation of T cells in mammals. Though IL-2 and IL-15 both interact with the xcex2 and xcex3 chains of the IL-2 receptor, and may share some elements of tertiary structure, the two polypeptides are not homologous and represent distinct gene products.
The genes encoding IL-15 from several different mammalian species share a high degree of homology. For example, human and simian IL-15 share 97% amino acid homology. By contrast, chicken IL-15, which is the subject of the present invention, shares only 25% amino acid identity with mammalian IL-15. Another distinguishing characteristic of chicken IL-15 is that it (and not the mammalian forms) is produced by mitogen-activated spleen cells. Accordingly, the discovery of chicken IL-15 and the finding that it possesses T cell-stimulatory activity provide a novel reagent for vaccine augmentation in avian species. Without wishing to be bound by theory, the bioactivity of mammalian IL-15 in stimulating skeletal muscle development suggests that avian IL-15s are also useful in stimulating growth in avian species.
The present invention provides isolated and purified DNA encoding avian interleukin-15 (IL-15), as well as cloning and expression vectors comprising IL-15 DNA and cells transformed with IL-15-encoding vectors. Avian species from which IL-15 may be derived include without limitation chicken, turkey, duck, goose, quail and pheasant.
The invention also provides isolated and purified avian IL-15 polypeptide, the native secreted or mature form of which has a molecular mass of about 14 kDa, an isoelectric point of about 6.57, a net charge of xe2x88x922, and a hydrophilicity index of 0.278, and which has the ability to stimulate mitogen-activated avian T cells and to promote the growth of other cell types. IL-15 according to the present invention may be obtained from native or recombinant sources.
Also encompassed by the invention are sequence-conservative and function-conservative variants of avian IL-15 DNA and IL-15 polypeptides, including, for example, a bioactive IL-15 sequence or sub-fragment that is fused in-frame to a purification sequence.
In another aspect, the invention provides a method for enhancing an immune response in fowl to an immunogen, which is achieved by administering the immunogen before, after, or substantially simultaneously with avian IL-15 in an amount effective to enhance the immune response.
In yet another aspect, the invention provides a vaccine for inducing an immune response in fowl to an immunogen, comprising the immunogen and an effective amount of avian interleukin-15 for immune response enhancement. The immunogen may be derived, for example, from avian pathogens such as Marek""s Disease Virus, Newcastle Disease Virus, Infectious Bursal Disease Virus, Infectious Bronchitis Virus, and the like.